This invention relates generally to pressure vessels and more particularly to an improved drain and venting system for pressure vessels which are used as steam cookers.
Steam or pressure-type cookers have been successfully employed by restaurants, hospitals and other food service operations to prepare quickly and conveniently large quantities of food. These steam cookers are made especially convenient for the preparation of frozen foods, by way of incorporating a defrosting step as an initial part of the total cooking process. Steam cookers of this type are well known in the art, as is illustrated by U.S. Pat. Nos. 3,071,063 and 3,071,473 issued to one George Churley. Generally, the steam cookers described in these patents operate by subjecting the food placed in the cookers to jets of pressurized steam. Because the steam jets impinge directly upon the food product and the food is subjected to a high pressure atmosphere of heat conducting steam, thawing and cooking are comparatively rapid in cookers of this type.
Typically, the drain system for prior art steam cookers has consisted of a drain pipe located in the bottom of the pressure treatment vessel and emptying into an open bell fitting which is connected with the building waste system. In this drain system a solenoid operated valve is arranged to enable and stop flow through the drain pipe.
In these prior art steam cookers the drain pipe or the drain valve or both are selected to have relatively small flow capacity and cross sectional areas, these cross sectional areas being limited in accordance with the flow capacity of the steam inlet system in order that differences in steam inflow and exhaust rates which result from the condensing of steam on frozen food can be detected. According to this method of automatic defrosting, once food in the cooker is unfrozen, the rate of steam condensation in the cooker slows noticeably and, as a result of the limited steam outlet aperture, steam pressure (and temperature) in the cooker begin to rise. In these prior art cookers this rise in steam pressure is sensed by a pressure responsive device or other steam energy sensing apparatus and the cooker drain valve is closed in response thereto. As a result of steam pressure in the vessel being related to the temperature (or vice-versa) either of these parameters can be sensed to close the drain valve. Since termination of the defrosting cycle and commencement of the cooking cycle in the vessel is dependent upon pressure buildup (or temperature rise) the vessel drain pipe and drain valve in these prior art cookers are necessarily of a size which is small enough to restrict steam outflow and allow the pressure to build in the vessel at the end of the defrost cycle.
Because the drain valve in an automatic defrost cooker is necessarily of this small size, it can readily become obstructed with food particles, which collect in the bottom of the vessel. (At least part of the food particles which fall from a cooking pan in these cookers is carried into the drain with exhausting steam, particularly at the end of a cooking cycle.) As a result of this defrosting cycle and the sensing of pressure vessel steam energy level, the existence of a clogged drain may render the cooker inoperative, since the defrost cycle is prevented from operating properly. It is therefore common that prior art steam cooker drain systems must be periodically disassembled, checked and cleaned of food particles that could plug the drain system, particularly the drain valve.
An obvious solution to this difficulty would be to simply replace the smaller drain valve by a larger one; however, such a substitution would alter the relationship between steam inflow and steam exhaust during the defrost cycle. Other factors remaining the same, a larger drain and drain valve would allow steam to continue to exhaust at about the same rate as supplied to the cooker, pressure would not build-up after the defrost part of the cycle, and the pressure sensitive switch would never sense a vessel pressure change and close the valve to complete the cooking part of the cycle.
Even the inclusion of an additional drain valve, e.g., as is shown in U.S. Pat. No. 3,804,591 to Bezrodny et al., would not prove to be a solution. The apparatus of Bezrodny et al is an autoclave having a single regulated exhaust which has two control valves in series in the exhaust to control the pressure in the autoclave, and a water drain valve in the bottom of the autoclave. Although the size and use of the water drain valve are not specified, it apparently is used only after the autoclave has been cooled down and depressurized. Such a system could only be used to sterilize sealed containers which would not produce spilled food particles. Further, it would require such a long period for depressurization and cool down that it would be useless for food service operation which requires the steam cookers to be quick and convenient, and the food to be hot.
A need therefore exists for an improved drain system for a steam cooker which will remain clear with little attention and still function as part of an automatic cook cycle.